Cells infected with cytomegalovirus (CMV, strain Colburn) contain large amounts of an intranuclear 51,000 dalton (51K) protein. This protein was dramatically enriched for by two different procedures used to identify DNA-binding proteins. The first of these demonstrated that the 51K protein was recovered following polyethyleneimine/high salt extraction of infected cells for proteins associated with DNA in vivo. And the second showed that, when analyzed by affinity chromatography using single-strand DNA coupled to Sepharose beads, the 51K protein was bound efficiently in low salt; was partially (40%) eluted at 0.30 M salt, along with "B-capsid" and "matrix" virus structural proteins; and constituted greater than 70% of the protein eluted at 0.45 M salt. Two-dimensional (charge-size) separations in denaturing polyacrylamide gels established that this acid soluble protein has a comparatively strong net positive charge; resolved the infected cell-specific 51K protein from a more neutrally charged host cell protein of about the same size; and verified observations that the virus-specific 51K protein is phosphorylated. In addition, results of biosynthetic radiolabeling experiments showed that expression of the 51K protein begins prior to that of most other viral proteins, but requires preceding viral protein and DNA synthesis.
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